The invention is directed to a method for reducing the levels of dioxins and other high molecular weight organic carcinogen emissions such as polychlorinated dibenzo furans generated by combustion processes into the atmosphere.
The presence of chlorinated dibenzo dioxins (PCDD) and chlorinated dibenzo furans (PCDF) has been reported in the emissions from municipal waste incinerators. They have been described as among the most acutely toxic substances known to man. Although the reported concentrations of these substances in incinerator emissions is low, the associated toxicity and carcinogenic implications are of substantial concern.
Earlier studies relating to PCDD and PCDF were directed to the identification and measurement of the PCDD and PCDF groups in the flyash collected by electrostatic precipitators (ESP). However, in the ASME Research Committee on Industrial and Municipal Wastes, Study on State of the Art of Dioxin from Combustion Sources (1980), the authors established that the dioxin collected with the flyash might be only a portion of the total dioxins emitted. They expressed the opinion that a considerable fraction of the PCDD might be associated with submicron particles and that notwithstanding all other factors, the focus on ESP flyash might not be sufficient for complete evaluation of PCDD emissions. An ESP is relatively more effective in removing large particles, i.e., those above submicron size. However, submicron particles are abundant in the flue gases emitted from the stacks of municipal waste incinerators. Moreover, submicron particulate show a much higher concentration of adsorbed organic pollutants than do larger diameter particulate materials.
The authors of the ASME study referred to preliminary data by Ballschmitter, who reported that the concentration of tetrachloro dibenzo dioxin (TCDD) in ESP flyash was significantly greater then the concentration of the TCDD in the flyash.
This type of phenomenon is similar to that referred to by Greenberg et al ES&T, 15(1), 64, (1981), who reported that more than 75% of the condensed compounds of Na, Cs, Cl, Br, Cu, Zn, As, Ag, Cd, Sn, Sb, W, and Pb were concentrated on particles with diameters less than 2 microns.
The authors of the ASME study further stated that variations in vapor phase concentration as a function of temperature may be responsible for part of the variation in PCDD group distributions from different facilities.
Data reported by Cavallaro A., et al., Chemosphere, II (No. 8), 859, (1982) for emissions from 6 incinerators in Italy and data reported by the Ministry of Environment of Ontario, Canada confirm the suspicions established in the ASME study. The dioxin concentration in flue gases are 10 to 1000 times greater than in the flyash and of that present in the flue gas, the concentration in the vapor phase can exceed that in the particulate phase.
The data reported by Cavallaro is condensed and appears in TABLE 1.
TABLE 1 ______________________________________ DIOXIN EMISSIONS IN FLUE GAS (EXCLUSIVE OF FLYASH) ##STR1## TCDD ng/Nm.sup.3 PCDD Inc. Va- ng/Nm.sup.3 No. por Part. Total V/p Vapor Part. Total V/p ______________________________________ 1 19.6 1.6 20.7 17.8 449.2 24.3 474 18.5 2 17 172.2 189 0.1 28751 20247 48998 1.4 3 19 0.037 19.0 513 7501 8.9 7510 842 4 60 10.9 70.9 5.5 4353 56.4 4409 77.2 5 9.6 0.34 9.6 28.2 648.6 381.6 1030 1.7 6 19 N.D. 19 -- 587 0.8 588 733 ______________________________________
The flyash collected generally contained significantly less dioxins than that present on the airborne particulate. A parallelism existed for the ratio of dioxin content of vapor to airborne particulate for the TCDD and PCDD. In all cases but one, the emissions of TCDD in the vapor phase significantly exceeded that on the particulate.
This distribution of the PCDD in the flue gas was confirmed by the Ministry of Environment, Ontario in tests at the Hamilton incinerator with the emissions, 25-30 ug/m.sup.3 measured after the flue gas passed through a precipitator.
Guidelines for ground level concentrations of the PCDD's have been established by the ministry of Environment, Ontario and the New York State Department of Environmental Conservation.
The Ontario guideline is: ##EQU1##
The New York guideline establishes a ground level of 9.times.10.sup.-14 gm/Nm.sup.3 for total TCDD as an annual average, based on the upper range of ambient concentrations included in the EPA's 1981 reportwhich that evaluated health implications of TCDD's from five municipal incinerators.
The projected maximum stack emissions based on these guidelines at different levels of dilution by dispersion are set forth in Table II.
TABLE II ______________________________________ MAXIMUM PERMISSIBLE STACK EMISSIONS FUNCTION OF STACK DILUTIONS Permissible Required Stack Concentration Pg/Nm.sup.3 ng/Nm.sup.3 at Dilution Factor ______________________________________ Dilution Factor 1000 10000 50000 Ontario PCDD 30 30 300 1500 N.Y.S. TCDD 0.09 0.09 0.9 4.5 ______________________________________
The dilution factors were based on the following (from a 100 m stack):
1000--fumigation maximum ground conc. PA1 10000--dispersion--Brookhaven Model B.sub.2, C, D. PA1 50000--annual averaging PA1 1--Thermal Destruction PA1 2--Capture and Containment.
The reductions in TCDD and PCDD emissions based on the average emissions reported by Cavallaro required to comply with the published guidelines are set forth in Table III.
TABLE III ______________________________________ PERCENT REDUCTION, CDD, REQUIRED TO COMPLY WITH GUIDELINES Percent Ontario New York PCDD at Dilution PCDD TCDD ______________________________________ 1000 99.0 99.7 5000 94.6 98.4 10000 89.3 96.8 ______________________________________
There two basic approaches to achieving the foregoing reductions are:
A 99.5% thermal destruction of dioxins has been reported to occur with exposure to a temperature of 1177.degree. C. It was noted that a residence time of 2-3 seconds at 1200.degree. C. provided effective destruction of PCB's in cement kilns. However, the reliable achievement of a combustion temperature of 1100-1200.degree. C. in the incineration of municipal waste is questionable, principally because of the heterogeneity and variation in water content of the fuel.
It was reported that Gizzi et al., Chemposphere, II(6), 577, (1982), that the TCDD total emissions varied widely in 17 incinerator samples with an average of 70 ng/m.sup.3 for 15 of the 17 and .about.1700 ng/m.sup.3 for 2 of the tests. The authors stated that for the two tests where high levels of dioxin were noted "on these days, the incinerator operating combustion temperature was the lowest, reaching about 500.degree. C, probably because of the high moisture content of the waste." This change in water content is unavoidable in municipal waste incineration and results in inconsistency of thermal destruction. Auxiliary fuel firing responsive to temperature needs can be used to achieve destruction of the PCDD and PCDF compounds. However, control reliability and costs can be prohibitive.
Furthermore, an increase in operating temperature above the normal 1000.degree. C. will result in an increase in NOx emissions, a counter productive measure, in view of NOx limitations, such as required in California. Although thermal destruction represents the most desirable mode of removal of the dioxin emission potential, there exists the necessity to prevent emission of residual dioxins present in the flue gas, because of the inherent lack of reliability of temperature control and the potential for increase in NOx formation.
The dioxins emitted from combustion processes are in the form of gas, submicron sublimates and are absorbed on other fine particulates. At the present time, conventional methods for the removal of dioxins are ineffective. Capture by electrostatic precipitators is inefficient because dioxins in the vapor form and the submicron form pass through the precipitators without collection. Higher efficiencies are achievable for particulate collection with a baghouse but the high concentration of fine particulates along with the dioxin results in excessive pressure drops and blinding of the bags. As mentioned above, maintaining high temperature when incinerating municipal waste is not feasable.